Large quantities of water are produced during the processing of hydrocarbons in offshore facilities. One example is in the production (removal) of hydrocarbons from subsea reservoirs by flowing the hydrocarbons up to a structure at the sea surface such as a floating vessel, a spar or floating tension leg platform (TPL), or a platform. Processing equipment on the sea surface structure separates the hydrocarbons from other material, which commonly consists primarily of water, and may include sand, etc. The large quantities of such produced water must be disposed of, either by injection into the reservoir (which is undesirable and costly) or by discharge into the environment. The produced water may be at an elevated temperature that is viewed by many as potentially detrimental to normal marine flora and fauna. Local regulations commonly require that large quantities of water such as the quantities commonly produced from undersea reservoirs, be cooled to a certain temperature before release into the sea.
In one example, water accompanying hydrocarbons from an undersea reservoir is at a temperature such as 90° C. (194° F.) and local regulations require that the temperature of discharged water be no greater than 40° C. (104° F.). Since the temperature of the sea is below that of hot water from the reservoir and the facility has ready access to sea water, it is logical to use sea water to cool the water from the reservoir. However, because of the large quantities of water that are produced (e.g. 1000 gallons per minute), the cost of conventional temperature-reduction heat equipment comprising sea water lift pumps, filters, heat-exchangers, etc. can be considerable. A cooling system with a minimal number of parts, which effectively cooled large quantities of produced water, would be of value.
There is a need for systems in the regassification of transported LNG (liquified natural gas), to heat cold water prior to its discharge into the sea. Gaseous hydrocarbons are commonly transported as LNG at −160° C. (−320° F.) if it contains methane, as LPG (propane and butane) at −50° C., or as hydrates (gas trapped in ice crystals) at −40° C., all at atmospheric pressure. Such gaseous hydrocarbons are offloaded, as directly into a gas pipeline whose outer end is located on a fixed or floating structure, so the gas can flow to shore and/or to an underground (under sea or shore) storage cavern for later use. The liquified gas is heated, as to 5° C. to avoid very cold pipes on which moisture condenses and to avoid cracking of walls of a salt dome cavern in which gas is stored. In this application it also is logical to use sea water to warm the very cold liquid to regas it. Local regulations may require that the temperature of large quantities of discharged water be at least 10° C. (50° F.).
In both the heating and cooling of produced water, local regulations require avoidance of “hot spots” or “cold spots” where marine life may be subjected to extreme temperatures. For examples, sea animals may be attracted to warm discharged water, and they must be protected from being burned as a result of a close approach to the location(s) where warm water is discharged into the sea. A system that changed the temperature of large quantities of discharged water to be closer to the temperature of the ambient or surrounding sea while avoiding “hot” or “cold” spots, and which used a low cost and effective system to accomplish this, would be of value.